Method and apparatus for handling metallic melts



Nov, 8, 1960 F. HARDERS 2, 59,478

METHOD AND APPARATUS FOR HANDLING METALLIC MELTS Filed Dec. 25, 1957 I5 Sheets-Sheet 1 InVem0/1' FR/TZ 'h'A koiks A 770K415 Y5 Nov. 8, 1960 F. HARDERS 2,959,478

METHOD AND APPARATUS FOR HANDLING METALLIC MELTS Filed Dec. 23, 1957 Y s Sheets-Sheet 2 Inventor F/?/ 72 HA #0565 A TTWP/VE/S Nov. 8, 1960 F. HARDERS 2,959,473

METHOD AND APPARATUS FOR HANDLING METALLIC MELTS Filed Dec. 23, 1957 3 Sheets-Sheet 3 .fm/enton' FRI 72 HA ROI-RS ATTO 0N5 Y5 United States Patent lVIETHOD AND APPARATUS FOR HANDLING METALLIC MELTS Fritz Harriers, Post Ergste uber Schwerte (Ruhr), Germany, assignor to Dortmund-Hinder I-Iiittenunion Aktiengesellschaft, Dortmund, Germany Filed Dec. 23, 1957, Ser. No. 704,736 Claims priority, application Germany Dec. 22, 1956 1 Claim. (Cl. 75--49) In the production of steel on a large technical scale, the metallurgical reactions are controlled by controlling the metal melt, the slag, and their composition, concentration and temperature.

The invention is based on the fact that the course of the metallurgical reaction is decisively determined by the carbon monoxide reaction. By making use of the fact that this latter is largely dependent on pressure, the invention achieves a surprising and very significant improvement in the production of steel on a large technical scale. In detail, it is proposed that the steel, which interacts at normal pressure with the slag floating on it, should be subjected from time to time to a reduced pressure. This reduction in pressure results in a very vigorous release of the carbon monoxide reaction which is decisive for the course of the whole process and for the final quality of the product and which is greatly dependent on pressure. The manufacturing process of steel is consequently controlled in a way which has hitherto not been possible.

According to the invention, the melt, which is at normal pressure in any customary metallurgical furnace, is raised in portions into a vacuum vessel disposed above the furnace, and left there for some time at the most favourable reduced pressure and then returned to the melt remaining in the furnace. These operations are repeated until the metal beneath the slag is of the desired composition. In this process, the melt, which is subjected to the necessary reactions between steel and slag in the furnace at normal pressure, has been altered in such a way that as to have a particularly favourable effect on the reactions between the slag and metal in the furnace. These reactions are further promoted to a decisive extent by the very vigorous agitation of the metal under the slag resulting from drawing the portions of the melt into the vacuum vessel and returning them again.

The process of the invention may, for example, be carried out with the apparatus shown in the drawings, which are part sectional views through three different forms of apparatus.

In Fig. 1 the process is carried out using an arc furnace.

A Hieroult-arc furnace 7 of the usual construction is equipped with a fore-hearth 1. A short pipe 2 leads from a vacuum vessel 3, which has a refractory lining and a cover 4 which has a vacuum tight seal, and extends into the forehearth. A suction pipe 5 leads to the vacuum pumps which are not shown. The reference 6 is a feeding device, e.g. for alloying materials. It is provided with an air lock.

When the pressure in the vacuum vessel 3 is lowered, a portion of the melt in the arc furnace is drawn through the pipe 2 into the vacuum vessel 3. The melt is returned to the charge remaining in the arc furnace, for example, by altering the pressure in the vacuum vessel. When the pressure is then again reduced, a fresh portion of the melt enters the vacuum vessel and undergoes the same treatment. Alternatively, one may fill and empty the vacuum vessel by altering the level of the vessel while maintaining the vacuum in it. At the great speed with which the carbon-oxygen reaction proceeds, the intermediate vacuum treatment of the metal takes place very rapidly. For example, it can be carried out in minute, including the raising and lowering of the vessel, when the pressure is in the region of 1-10 mm. Hg. In the case where the vacuum vessel has, for example, the capacity of the furnace, then the vacuum treatment of the entire melt is completed in 7 /2 minutes. Owing to the backwards and forward flow which it produces, the vacuum vessel functions at the same time as a kind of stirring apparatus.

If the pipe 2 is of an appropriate shape, the forehearth can be dispensed with and the pipe 2 leading off from the vacuum vessel can in that case be passed through a door of the furnace, as shown in the apparatus in Fig. 2. The vacuum vessel may also form a single unit with the furnace.

Any kind of heated open hearth furnace, for example a Siesmens Martin furnace, may be equipped in the same way as the arc furnace with a vacuum installation.

The vacuum installation which operates in conjunction with the furnace may of course also be provided with heating means as, for example, are heating or carbon resistance heating.

The advantages of the process of the invention for treating steel melts will be explained using arc melting as example:

In the refining process, the vacuum installation can be used very effectively as a refining unit, for example by refining to the blank metal in the low pressure chamber by addition of ore.

Since the equilibrium values for the oxygen in dependence on the particular low pressure used and on the carbon content of the melt are established in a very short time, the usual, time-consuming fining period which might otherwise be necessary may be omitted, unless a white lime or a carbide slag is desired in view of the sulphur content.

Owing to the great dependence of the carbon monoxide reaction on pressure, it is possible to obtain, according to the degree of vacuum used, a very low content in carbon together with very low oxygen contents. In such cases, the vacuum apparatus can at the same time be used as a measuring instrument. As long as there is an excess of carbon in the melt, addition of oxygen causes carbon monoxide to be given off, and this leads to a measurable increase in pressure. The same reaction takes place when carbon is added in the presence of excess oxygen.

It is no longer necessary to preheat additions and alloying constituents to protect the steel from taking up of hydrogen. Similarly, owing to the vigorous vacuum degasification, it is unnecessary to carry out the refining step which is normally carried out on remelting charges in some cases, merely for the purpose of removing gas, in particular hydrogen.

Inexpensive alloy carriers, which have a high carbon content, may also be used when the final product is required to have a very low carbon content. If appropriately low pressures are employed, the carbon can be refined out, in the presence of almost any alloying elements without these elements slagging. This refining may be done, for example, by introducing ores into the vacuum vessel. This is also of particular importance for highly alloyed remelting charges with a particularly low final carbon contents, the production of which in a vacuum plant with an arc furnace coupled to it provides no difliculty.

The usefulness of the Siemens-Martin process is also considerably increased by including the vacuum treatment. Apart from the possibility of refining in the vacuum vessel, the refining process itself is considerably promoted by the vigorous stirring effect produced by the vacuum treatment. The previously necessary time consuming step of waiting for the boiling reaction to die down can now be omitted, because the oxygen content of the melt is controlled by the vacuum treatment.

The reducing action of the melt which is poor in oxygen as a result of the vacuum treatment can be utilised for lowering the iron content in the slag. vFor this purpose, the furnace atmosphere is adjusted to be reducing, so that it does not introduce any further significant amounts of oxygen to the slag. At the same time, the vacuum apparatus continues to operate and keeps the oxygen content, by reaction with the carbon, as low as corresponds to the reduced pressure used. The reducing influence of the steel melt may cause the iron content of the slag to fall below the equilibrium value corresponding to atmospheric pressure, because the oxygen content of the melt is determined by the low pressure prevailing in the vacuum vessel.

Apart from the reduction in the iron waste, a slag poor in iron and therefore also in oxygen is also important in order that the waste of alloying agents during the addition in the furnace may be kept low.

Extremely low carbon contents together with very low oxygen contents can also be achieved under the conditions of the Siemens-Martin furnace by associating with it a vacuum installation and the facilities for measuring may be utilised in the same way as described for electric furnaces. Any adjustment can also be made by addition of carbon and oxygen to the vacuum vessel, in cases where the desired end value was not obtained or if the additional supply of oxygen from the oxidising slag must be compensated or neutralised.

In finishing the melt, the attached vacuum installation is valuable for adding carbon and alloying elements without loss to the melt by introducing them into the vacuum vessel. The process described of carrying outa vacuum treatment of a melt while at the same time carrying out slagging at atmospheric pressure can of course be applied to any steel manufacturing process, because the carbon monoxide reaction, which is greatly dependent on pressure, always plays a decisive part. Also, the thorough degasifying effect of the low pressure treatment always leads to a significant improvement in quality.

In addition to the above mentioned processes, there is always the advantage of the vigorous stirring by the vacuum unit, leading to an accelaration of all the reac tions and an improved transfer of heat to the melt, as well as a more rapid solution of the alloying additions and their even distribution in the melt.

The process of the invention also has greatadvantages in the known refining of pig iron by blowing air or oxygen onto the surface of the melt,

Fig. 3 shows a constructional example of an open hearth refining furnace in which oxygen is blown on to the surface of the melt. The vacuum vessel 2 isarranged on the end face of the furnace 1, where its suction pipe 3 dips into the forehearth 4. The reference 5 is the oxygen inlet passing through the pipe 6.

The mode of operation will be described in connection with the refining of pig iron rich in phosphorous as an example.

As is known, dephosphorisation precedes decarburisation in the desired manner if the oxygen is supplied substantially above the slag, i.e. if it is blown-onto the slag and not onto the melt. Furthermore, there must be adequate stirring of the melt in order to ensure that the reaction between the slag and the metal melt is sufiicient. Admittedly, when oxygen is blown only onto the slag, a certain amount of stirring takes place owing to thesimultaneous combustion of carbon which takes place to some extent. It has, however, been found that the combustion of carbon under these conditions cannot be controlled with as much certainty as is necessary for the operation.

Also, towards the end of the dephosphorisation, in some cases, phenomena which are similar to delayed boiling phenomena in the carbon-oxygen react have been observed which result in vigorous expulsion and disturb the course of the operation. These difficulties do not occur in the process described. The to and fro movement of the melt during the operation of the vacuum vessel produces an agitation of the bath which is adequate for rapid dephosphorisation and which avoids the abovementioned phenomena with certainty.

In the methods of top air refining customarily used nowadays, the slag becomes rich in iron towards the end of the refining process when the carbon content becomes low, and these high contents in iron in the slag are harmful to the lining and have an unfavorable effect on production. The process of the invention produces more favourable results by interposing a vacuum process. As the melt is poor in oxygen even when the carbon content is low at the low pressure prevailing in the vacuum vessel, it has a reducing action on the slag and lowers its iron content, this process being further promoted by a vigorous agitation of the melt. When the supply of oxygen is completely cut off towards the end of the refining process, so that no further significant supply of oxygen from the gaseous atmosphere to the slag occurs, the reducing action of the melt which is poor in oxygen on the slag will be proportionately more marked. It is also possible in the present case that the iron reduction from the slag exceeds the equilibrium values for atmospheric pressure, since the oxygen content of the melt corresponds to the low pressure in the vacuum vessel.

The obtaining of the lowest carbon values with simultaneously very low oxygen values is of course just as possible in the top air refining process with the apparatus described as in the Siemens-Martin process. The same applies to the other advantages mentioned in connection with the Siemens-Martin process.

The advantages described for the top air refining of types of pig iron rich in phosphorus apply also to top air refining of other kinds of pig iron. The use of technically pure oxygen, i.e. oxygen free from nitrogen, is not necessary in an air refining process carried out in accordance with the invention.

I claim:

A process for the metallurgical treatment of metals, particularly iron and steel melts, in which a slag cover suitable for the metallurgical treatment of metal is produced on the metal in the melt furnace and in which an adjustable vacuum chamber under a vacuum, located above the content of said melt furnace, adjustable as to height and provided with a pipe outlet dipping into the metal melt, is first lowered towards the bath level of the metal melt, until it is filled with a portion of the metal under the'slag cover, the chamber being then raised and the metal located therein leaving said pipe outlet and en tering into the metal under the slag with a velocity sufficient to cause a vigorous agitation, the raising and lowering of the vacuum chamber being repeated several times for the purpose of aiding the metallurgical treatment of the metal under the slag cover.

References Cited in the file of this patent UNITED STATES PATENTS 1,921,060 Williams Aug. '8, 1933 FOREIGN PATENTS 569,699 Germany Feb. 6, 1933 138,020 Austria June 25, 1934 

